In an aircraft gas turbine engine fuel supply system it is common to use a positive displacement pump such as a gear pump or a piston pump to provide fuel at a high pressure through a fuel metering unit (FMU), to the burners of the gas turbine engine. Positive displacement pumps all have an individual internal leakage characteristic in that some proportion of the high pressure output flow of the pump leaks back to the low pressure inlet side of the pump by way of internal leakage flow paths in the pump.
Such leakage can become progressively worse with time as a result of wear of the internal components of the pump. Further, pumps will wear at different rates dependent upon their usage and the ambient conditions in which they operate. Therefore, eventually a point is reached in the service life of every pump at which the leakage is sufficient to prevent the pump from being able to deliver either the flow required to start the associated engine at low engine/pump speeds, or provide the flow required for aircraft take-off at maximum engine/pump speed. Further, the engine fuel pump is often integral with the fuel control so in those situations there is no practical way to assess the performance of the fuel pump by, for example, measuring actual pump fuel flow. Accordingly, currently fuel pumps are replaced on a set schedule that is based on a worst case scenario so it is often the case that fuel pumps are replaced that still have remaining useful life.
Accordingly, it is desirable to provide a prognostic algorithm for predicting when a fuel pump maintenance action will be required.